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| United States Patent |
5,109,441
|
|
Glaab
|
April 28, 1992
|
Fiber optic external modulator
Abstract
An improved external optical modulator provides reduced noise and
distortion. An optical carrier to be modulated is split into a plurality
of portions. A first portion of the carrier is modulated with an
information signal. A second portion of the carrier is processed to
provide an attenuating signal. The modulated carrier portion is combined
with the attenuating signal to provide an attenuated optical carrier
having improved apparent percentage modulation. In a preferred embodiment,
the first carrier portion comprises a substantially greater amount of
optical carrier power than the second carrier portion.
| Inventors:
|
Glaab; Joseph B. (New Hope, PA)
|
| Assignee:
|
General Instrument Corporation (Hatboro, PA)
|
| Appl. No.:
|
642821 |
| Filed:
|
January 18, 1991 |
| Current U.S. Class: |
385/1; 385/3 |
| Intern'l Class: |
G02B 006/10 |
| Field of Search: |
350/96.11,96.12,96.13,96.14
|
References Cited
U.S. Patent Documents
| 4683448 | Jul., 1987 | Duchet et al. | 332/7.
|
| 4684207 | Aug., 1987 | Lawless | 350/96.
|
| 4694276 | Sep., 1987 | Rastegar | 340/347.
|
| 4758060 | Jul., 1988 | Jaeger et al. | 350/96.
|
| 4763973 | Aug., 1988 | Inoue et al. | 350/96.
|
| 4763974 | Aug., 1988 | Thaniyavarn | 350/96.
|
| 4776657 | Oct., 1988 | Reeder | 350/96.
|
| 4850667 | Jul., 1989 | Djupsjobacka | 350/96.
|
| 4878723 | Nov., 1989 | Chen et al. | 350/96.
|
| 4932738 | Jun., 1990 | Haas et al. | 350/96.
|
| 4936644 | Jun., 1990 | Raskin et al. | 350/96.
|
| 4936645 | Jun., 1990 | Yoon et al. | 350/96.
|
Other References
L. M. Johnson and H. V. Roussell, "Reduction of Intermodulation Distortion
in Interferometric Optical Modulators", Optics Letters, vol. 13, No. 10,
Oct. 1988, pp. 928-930.
|
Primary Examiner: Gonzalez; Frank
Attorney, Agent or Firm: Lipsitz; Barry R.
Claims
What is claimed is:
1. An optical modulator for modulating an optical carrier with an
information signal comprising:
means for splitting an optical carrier received from a first source into a
plurality of portions;
means, coupled to receive a first portion of said optical carrier from said
splitting means, for modulating said first portion with an information
signal received from a second source;
means, coupled to receive a second portion of said carrier from said
splitting means, for processing said second portion to provide an
attenuating signal for use in attenuating said optical carrier; and
means, coupled to receive said attenuating signal and the modulated first
portion of said optical carrier, for combining the modulated first portion
with said attenuating signal to provide an intensity modulated attenuated
optical carrier.
2. An optical modulator in accordance with claim 1 wherein:
said first portion of said optical carrier is modulated in a first path of
a Mach Zehnder type modulator to which said information signal is applied
at one polarity;
said second portion of said optical carrier is phase shifted in a second
path of said modulator to which a bias signal is applied at said one
polarity;
a third portion of said optical carrier is modulated in a third path of
said modulator complementary to said first path and to which said
information signal is applied at a polarity opposite to said one polarity;
and
a fourth portion of said optical carrier is phase shifted in a fourth path
of said modulator complementary to said second path and to which said bias
signal is applied at said opposite polarity.
3. An optical modulator in accordance with claim 2 wherein said combining
means comprise an interferometer coupled to receive said first, second,
third and fourth portions of the optical carrier.
4. An optical modulator in accordance with claim 3 wherein said splitting
means comprise an optical power splitter for providing about one-third of
the total optical carrier power to each of said first and third paths and
about one-sixth of the total optical carrier power to each of said second
and fourth paths.
5. An optical modulator in accordance with claim 2 wherein said splitting
means comprise an optical power splitter for providing about one-third of
the total optical carrier power to each of said first and third paths and
about one-sixth of the total optical carrier power to each of said second
and fourth paths.
6. An external optical modulator comprising:
first and second optical paths adapted to receive and carry first equal
portions of an optical carrier;
means operatively associated with said first and second paths for equally
and oppositely phase modulating said carrier portions in said first and
second paths, respectively;
third and fourth optical paths adapted to receive and carry second equal
portions of said optical carrier;
biasing means operatively associated with said third and fourth paths for
equally and oppositely phase shifting the carrier portions in said third
and fourth paths, respectively; and
means coupled to receive and combine the modulated carrier portions from
said first and second paths with the phase shifted carrier portions from
said third and fourth paths for providing an intensity modulated output
signal.
7. A modulator in accordance with claim 6 wherein said combining means
comprise an interferometer coupled to combine the carrier portions from
said first, second, third and fourth paths.
8. A modulator in accordance with claim 6 wherein said first equal portoins
each comprise about one-third of the power of said optical carrier and
said second equal portions each comprise about one-sixth of the power of
said optical carrier.
9. A method for externally modulating an optical carrier to communicate an
information signal over an optical signal distribution path, comprising
the steps of:
splitting an optical carrier to be modulated into a plurality of portions;
modulating a first portion of said carrier with an information signal;
processing a second portion of said carrier to provide an attenuating
signal; and
combining the modulated carrier portion with said attenuating signal to
provide an attenuated optical carrier having improved apparent percentage
modulation.
10. A method in accordance with claim 9 wherein said first carrier portion
comprises a substantially greater amount of optical carrier power than
said second carrier portion.
11. A method in accordance with claim 10 wherein said first carrier portion
comprises about two-thirds of said optical carrier power and said second
carrier portion comprises about one-third of said power.
12. A method in accordance with claim 9 wherein said processing step shifts
the phase of said second carrier portion.
13. A method in accordance with claim 12 wherein said phase is shifted by a
fixed amount.
14. A method in accordance with claim 9 wherein said processing step
reduces the amplitude of said second carrier portion.
15. A method in accordance with claim 14 wherein said amplitude is reduced
by a fixed amount.
Description
BACKGROUND OF THE INVENTION
The present invention relates to optical modulators, and more specifically
to a technique for reducing noise and distortion in the output of an
external optical intensity modulator.
Recently, there has been a growing interest in the development of analog,
amplitude modulated optical communication systems. In comparison with
digital systems, analog communication systems provide an efficient use of
bandwidth. This is particularly useful in cable television (CATV)
transmission system applications, where it is necessary to transmit a
large number of video channels through an optical fiber. Compatibility
with existing equipment is achieved by using the same signal format for
optical transmission that is in use for coaxial cable signal transmission.
In order to transmit an information signal (e.g., a television signal) over
an optical fiber, a light beam ("carrier") must be modulated with the
information signal. The "electrooptic effect" has been advantageously used
to provide modulators for this purpose. For example, electrooptic
modulators using miniature guiding structures are known which operate with
a low modulating power.
In electrooptic modulators, the electric field induced linear birefringence
in an electrooptic material produces a change in the refractive index of
the material which, in turn, impresses a phase modulation upon a light
beam propagating through the material. The phase modulation is converted
into intensity modulation by the addition of polarizers or optical
circuitry. Ideally, an electrooptic modulator should have a linear
relationship between its output optical power and the applied modulating
voltage.
In a "Mach Zehnder" type electrooptic modulator, an optical carrier (laser
beam) is split into two paths. At least one path is electrically phase
modulated. The two signals are then recombined in an interferometer to
provide an intensity modulated carrier. Typically, lithium niobate
(LiNbO.sub.3) is used as the electrooptic material. Waveguides in such
materials are readily formed by titanium indiffusion.
The output power curve of a Mach Zehnder modulator is nonlinear. Practical
analog optical communications systems, however, demand a high linearity.
See, for example, W. I. Way, "Subcarrier Multiplexed Lightwave System
Design Considerations for Subscriber Loop Applications", J. Lightwave
Technol., Vol 7, pp. 1806-1818 (1989). Modulator nonlinearities cause
unacceptable harmonic and intermodulation distortions. When it is
necessary to communicate a large number of channels, as in a CATV
application, intermodulation distortions ("IMD") can impose serious
limitations on the system performance. In principle, the second order IMD
can be filtered out if the bandwidth is less than one octave. However,
CATV transmission systems operate with bandwidths of many octaves. Third
order IMD can only be eliminated by using devices with linear
characteristics.
Another type of external optical modulator is the acoustooptic modulator.
In these devices, the phase grating created by an acoustic wave through
the photoelastic effect can either diffract a light beam into many orders
as in the Raman-Nath regime of operation or deflect a light beam into a
single order as in the Bragg regime. In either regime, intensity
modulation of moderate bandwidth is easily accomplished without regard to
the polarization of the incident light. At present, the bandwidth of
acoustooptic modulators is limited to about a few hundred megahertz by
practical considerations of the high frequency transducer design.
Guidelines for the selection of acoustooptic materials for device
applications are discussed in D. A. Pinnow, "Guidelines for the Selection
of Acoustooptic Materials", IEEE J. Quantum Electron., Vol. QE-6, pp.
223-238, Apr. 1970. A review of acoustooptic materials and techniques for
light deflection is presented by N. Uchida and N. Niizeki, "Acoustooptic
Deflection Materials and Techniques", Proc. IEEE, Vol. 61, pp. 1073-1092,
Aug. 1973. Acoustooptic modulators also exhibit a nonlinear relationship
between output optical power and the applied modulating voltage. As a
result, IMD must be reduced to provide practical operation in applications
such as cable television transmission.
Typical CATV fiber optic systems using frequency division multiplexed
amplitude modulated (AM-FDM) signals will modulate the light output of a
laser diode proportionally to the composite AM signal of the cable
television FDM spectrum. Lasers with adequate power output and low
distortion are expensive and difficult to make. An alternate scheme is to
use a high power laser and externally modulate the laser beam. As noted
above, known external modulators are nonlinear, although a small linear
range of operation is generally available. In order to operate such
modulators with low distortion, a high optical carrier input power and
small depth of modulation must be used over the limited linear range. When
a high power optical signal is output for transmission, the receiving
diode yields a large shot noise product. This, coupled with the low
modulation percentage, gives a lower than desirable signal to noise ratio
in the receiver.
It would be advantageous to provide an external optical modulator that
reduces the nonlinear distortion, and particularly second order
distortion, of the modulated signal. It would be further advantageous to
provide such a modulator that outputs a reduced optical carrier power, to
increase the effective modulation of individual carriers and reduce the
receiver shot noise. The present invention provides an external optical
modulator having the aforementioned advantages.
SUMMARY OF THE INVENTION
In accordance with the present invention, an optical modulator having
reduced noise and distortion is provided. An optical carrier is split into
a plurality of portions. A first portion of the optical carrier is
modulated with an information signal. A second portion of the carrier is
processed to provide an attenuating signal. The modulated and processed
carrier portions are combined to provide an intensity modulated attenuated
optical carrier.
The present invention can be implemented in a balanced Mach Zehnder type
modulator wherein the first portion of the optical carrier is modulated in
a first path to which the information signal is applied at one polarity.
The second portion of the optical carrier is phase shifted in a second
path to which a bias signal is applied at said one polarity. A third
portion of the optical carrier is modulated in a third path complementary
to the first path. The information signal is applied to the third path at
a polarity opposite to said one polarity. A fourth portion of the optical
carrier is phase shifted in a fourth path complementary to the second
path. The bias signal is applied to the fourth path at said opposite
polarity.
In the balanced Mach Zehnder modulator embodiment, the combining means can
comprise an interferometer coupled to receive the first, second, third and
fourth portions of the optical carrier. The splitting means in the
balanced Mach Zehnder modulator embodiment can comprise an optical power
splitter for providing about one-third of the total optical carrier power
to each of the first and third paths, and about one-sixth of the total
optical power to each of the second and fourth paths.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of a prior art Mach Zehnder type external
modulator;
FIG. 2 is a schematic diagram of a double balanced Mach Zehnder type
modulator with carrier level suppression in accordance with the present
invention;
FIG. 3 is a graph showing the output waveforms of a Mach Zehnder modulator
of the type shown in FIG. 1;
FIG. 4 is a graph showing the output waveforms of a balanced Mach Zehnder
modulator without carrier level suppression; and
FIG. 5 is a graph showing the output waveforms from an external optical
modulator with carrier level suppression in accordance with the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides a method and apparatus for reducing the shot
noise effects of an optical modulator and reducing the inherent distortion
of signals in an optical AM-FDM communication system.
In a conventional Mach Zehnder modulator such as that generally designated
10 in FIG. 1, a laser beam input to waveguide 12 is split into two paths
14, 16. Path 14 is electrically phase modulated by an input signal (e.g.,
an RF television signal) applied at terminal 26. The resultant electric
field across electrodes 22, 24 produces a change in the refractive index
of the waveguide 14, thereby phase modulating the portion of the laser
beam propagating therethrough. The phase modulated light is combined with
the light traveling through path 16 in an interferometer 18 that adds the
signals when they are in phase and subtracts the signals when they are out
of phase, producing an intensity modulated signal for output over optical
path 20. If the modulating path 14 is biased halfway, a plus/minus linear
range can be achieved. Correct biasing can provide a fairly good even
order distortion cancellation. However, the linear range over which the
modulator operates is small, requiring the power of the input laser beam
(optical carrier) to be high and a small depth of modulation to be used.
In such a system, shot noise is a problem at the receiver due to the high
power of the output signal. The large shot noise product and low
modulation percentage together cause the signal to noise ratio at the
receiver to suffer.
In accordance with the present invention, an optical modulator is provided
that reduces the overall output power as well as the distortion of the
phase modulation. A first portion of an optical carrier is modulated with
an information signal. A second portion of the carrier is processed to
provide an attenuating signal that is combined with the modulated portion
of the optical carrier to reduce the amplitude of the carrier with respect
to the modulation depth. This results in an intensity modulated attenuated
optical carrier that enjoys lower distortion and reduces shot noise at the
receiver.
One embodiment of the present invention is illustrated in FIG. 2. A
balanced Mach Zehnder modulator 28 receives an optical carrier via path 30
and splits the carrier into four paths 32, 34, 36 and 38. Paths 32, 34 are
similar to the conventional Mach Zehnder paths 14, 16 illustrated in FIG.
1. However, both paths 32 and 34 receive the modulating signal via
respective electrodes. For path 32, the modulating signal, which can
comprise an RF input signal and DC bias, is input at terminal 52. As a
result, an electric field is provided across path 32 by electrodes 50, 54.
This field modulates the optical carrier portion traveling through path
32.
The portion of the optical carrier passing through path 34 is similarly
modulated with an equal but opposite modulating signal input at terminal
72. This provides an electric field across electrodes 68, 70. The
structure of Mach Zehnder paths 32, 34 with the accompanying electrodes
and modulating signals provides a balanced modulator that operates in an
improved push-pull mode. The improvement is demonstrated by comparing the
graphs of FIGS. 3 and 4.
FIG. 3 illustrates the output of the conventional Mach Zehnder modulator of
FIG. 1. Curve 82 is the nominal carrier passing through the modulator.
Curve 80 illustrates the amplitude resulting from a -45.degree. phase
shift induced by the modulating signal. Curve 84 illustrates the amplitude
resulting from a +45.degree. phase shift induced by the modulating signal.
As is clear from FIG. 3, both the amplitude and phase of the carrier are
changed by the modulation.
FIG. 4 illustrates the output of a balanced Mach Zehnder modulator wherein
both paths are modulated with equal but opposite signals. Curve 88 is the
nominal carrier passing through the modulator. Curve 86 illustrates the
change in amplitude caused by a -45.degree. phase shift induced by the
modulating signal. Curve 90 illustrates the amplitude resulting from a
+45.degree. phase shift induced by the modulating signal. As can be seen,
the modulating signal in a balanced Mach Zehnder modulator causes the
amplitude of the output signal to vary (amplitude modulation) but does not
affect the output signal phase.
Turning back to FIG. 2, the structure of the present invention also
provides additional paths 36, 38 to provide carrier cancellation in
addition to modulation. In path 36, electrooptic phase control of the
optical carrier is provided by applying a DC bias signal to terminal 58.
The bias signal results in an electric field across electrodes 56, 60.
Similarly, the same bias voltage but at opposite polarity is input to
terminal 66 for providing an opposite electric field from electrode 62 to
electrode 64 across path 38. The DC bias applied to terminals 58 and 66
causes a phase shift in the optical carrier portions passing through paths
36 and 38, respectively. The modulated optical carrier portions from paths
32, 34 are then combined with the DC phase shifted portions of the optical
carrier from paths 36, 38 in an interferometer 40 to provide an output
signal on path 42 that enjoys an improvement in apparent percentage
modulation. This result occurs due to the cancellation of some of the
optical carrier power by the phase shifted portions from paths 36, 38. It
should be appreciated that although the overall average carrier level is
reduced at the output, the instantaneous (i.e., sideband) carrier power is
not reduced. By reducing the average carrier level, a higher apparent
modulation percentage is achieved.
In a preferred embodiment, splitter 31 is an optical power splitter that
provides approximately one-third of the total optical carrier power from
path 30 to each of modulation paths 32, 34. The remaining optical power is
evenly split between paths 36 and 38, so that each of these paths receives
about one-sixth of the total optical carrier power.
The resultant output signals are illustrated in FIG. 5. As can be seen, the
balanced operation of a modulator in accordance with FIG. 2 does not
induce a phase shift between various components of the output signal.
Curve 92 illustrates the amplitude of the output signal resulting from a
-45.degree. phase shift induced by the modulating signal applied to
terminals 52, 72. Curve 94 illustrates the normal amplitude of the optical
carrier. Curve 96 illustrates the amplitude of the optical carrier
resulting from a +45.degree. phase shift induced by the modulating signal.
Curves 92, 94 and 96 are equivalent to curves 86, 88 and 90 of FIG. 4.
The improvement provided by the present invention is illustrated by curves
98, 100 and 102. Curve 100 represents the optical carrier without
modulation, but reduced by providing a DC bias voltage at terminals 58, 66
sufficient to shift the phase of the carrier portions passing through
paths 36, 38 by 3 .pi./4. The required bias voltage can be determined
empirically, for example by tuning the modulator during its operation.
Curve 98 represents the amplitude of the reduced carrier when a modulating
signal is applied that shifts the phase in modulating paths 32, 34 by
-45.degree.. Curve 102 represents the amplitude of the reduced carrier
when the modulating signal provides a +45.degree. phase shift.
As can be seen by comparing curves 98, 100 and 102, an essentially 100%
depth of modulation can be achieved with only .+-.45.degree. of desired
modulation phase shift. This is a substantial improvement over prior art
optical modulators. The improvement is achieved in a manner which does not
create a phase shift of the light with resultant chromatic distortion
(dispersion) of the optical output signal. By reducing the absolute
carrier level transmitted to the receiver, the shot noise at the receiver
is reduced.
The fabrication of the device illustrated in FIG. 2 can be accomplished
using conventional techniques well known in the art of electrooptic
modulators.
It should now be appreciated that the present invention provides an optical
modulator providing an improvement in apparent percentage modulation as a
result of the cancellation of some of the light passing through the
modulator. This reduces the carrier level, increasing the percentage
modulation, while at the same time reducing shot noise induced at a
receiver by the modulated carrier. Although the invention has been
described in connection with various embodiments, those skilled in the art
will appreciate that numerous adaptations and modifications may be made
thereto without departing from the spirit and scope of the invention as
set forth in the following claims.
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